Shell operations: redirection, pipes, environment variables, grep, sed, awk, cut, sort, uniq, tr

The Linux shell's real power comes from combining small tools. Each program reads text, transforms it and writes text, and redirection and pipes connect them into a pipeline. Linux+ performance-based questions often ask you to build or interpret one of these one-liners, so read them left to right and ask what each stage receives and passes on. Every process has three standard streams: stdin (standard input, file descriptor 0), stdout (standard output, 1) and stderr (standard error, 2). > redirects stdout to a file, overwriting it; >> appends; < feeds a file to stdin. 2> redirects errors, 2>/dev/null discards them, and &> or > file 2>&1 sends both output and errors to the same file. Order matters: cmd > out 2>&1 works, while cmd 2>&1 > out sends errors to the terminal. A pipe | connects one command's stdout to the next command's stdin, and tee file copies the stream to a file while passing it on. A here-document (<<EOF) feeds inline text as input.